The present invention relates to optical copying machines in general, and more particularly to improvements in copying machines wherein the imaging system moves relative to the support for an original, or vice versa, and includes means for exposing successive strip-shaped portions of the original to modulated light in order to image such portions onto a photosensitive carrier, e.g., a web of photographic paper. The exposing means of the imaging system includes a mask or diaphragm which shields the remainder of the original from copying light.
When the imaging system of a copying machine includes one or more light sources (e.g., fluorescent lamps) which are in circuit with a source of alternating current, the intensity of copying light varies at a certain frequency whereby the reproduced image and its background often exhibit alternating rib- or strip-shaped regions (known as ripples or streaks) of more and less pronounced density which extend at right angles to the direction of movement and all the way between the marginal portions of the illuminated area of the carrier. Such undesirable formations are invariably attributable to fluctuations in the intensity of modulated copying light; they are especially pronounced if the number of light modulations during travel of a given portion of the original with respect to the imaging system or vice versa is relatively small. The just described situation will arise when the intensity of copying light is high, when the sensitivity of the carrier is high, when the carrier is a relatively narrow web and/or when the speed of the original with respect to the imaging system or vice versa is high.
It has been found that the development of ripples is attributable to rather pronounced differences between total amounts of light which impinge upon successive portions of the original during movement of the original relative to the imaging system or vice versa. In other words, and depending on the phase of the imaging system at the start of exposure of a given portion of the original to copying light, the timing of starting and final impulses of the imaging system is not identical for different portions or strips of the original.
My copending application Ser. No. 740,370 discloses an optical copying machine wherein the likelihood of the development of ripples on the copies of originals is reduced by selecting the width of successively illuminated strip-shaped portions of the original in such a way that the width is a whole multiple of the quotient of the speed of movement of the original with respect to the imaging system (or vice versa) and the modulation frequency of copying light. This insures that the amounts of light which impinge upon successive strip-shaped portions are always the same, independently of the phase of the imaging system at the start of illumination of a strip-shaped portion. However, the condition that the width of each strip-shaped portion of the original be a whole multiple of the aforementioned quotient can be met only if the relative speed between the original and the imaging system is constant or fluctuates within an extremely narrow range. It must be borne in mind that, if the gradation of the photosensitive carrier is steep, the copy will exhibit readily discernible ripples even if the amount of copying light fluctuates by less than 5 percent. Moreover, and since the intensity of light issuing from an elongated light source (e.g., a tubular fluorescent lamp) decrease from the center toward both ends, it is customary to use a diaphragm or mask with an elongated aperture whose width increases from the center toward both ends so as to compensate for the aforementioned characteristic of light sources. The aperture extends transversely of the original and determines the width of those strip-shaped portions of the original which are illuminated when the original moves relative to the mask and light source or vice versa. The fact that the width of the aperture increases from the center toward both ends affects the configuration of successively illuminated strip-shaped portions of the original. Deviations between the ideal configuration of such strip-shaped portions and the actual configuration (due to varying width of the aperture) are often so pronounced that the copy of the original exhibits streaks or ripples each of which varies in width as considered at right angles to the direction of movement of the carrier. Each ripple may but need not consist of two discrete halves which are disposed at the opposite sides of the central longitudinal symmetry plane of the carrier. In the case that each ripple consists of two substantially mirror symmetrical halves, one of the halves increases and decreases in width in a direction parallel to the longitudinal extension of the ripples. Such ripples of varying width develop when the width of the aperture of the mask decreases in directions from both ends toward the center of the aperture. The frequency of increase and decrease of the width of the ripples in the direction longitudinal to the respective ripple increases with increasing rate at which the width of the aperture increases from the center toward both ends thereof.
As a rule, each ripple extends across the entire width of the illuminated area. If the width of the aperture is constant, the width of the ripples is also constant. In each instance, the ripples are equally spaced apart from each other in the direction of movement of the carrier.